Who says? Each to their own. When listening quietly the human ear doesn't pick up bass so well, so a tone tweak can help improve the music...in my opinion
Two wrongs don't make a right...
Let me explain - listening to music is not just momentary experience it is a long process.
Life-lasting process.
During that process you grow. Your taste grows and your ears adapt and begin to perceive different details and tones, the pleasure from music grows with time (as opposed to other domapine induced gusto's).
So basic rule in selecting your gear is to get a system which will allow for this progress to happen, to help develop your musical taste and senses. Over time you should compound you pleasure from music and get more and feel more personal with it and closer to it.
If you drink coffee with 5 spoons of sugar you will never begin to enjoy the taste of coffee itself or the nuances b/w different coffees. Some people eat too much chili and argue this is how you better feel the taste of the food, but actually the opposite is true.
And same with music, you should never add distortion deliberately to such extreme levels. You cannot make love to the music if you rape it first.
In the case of "Loudness" it is crazy amount of distortion and definitely something one should not get used to. The long-term effect will be damaging your perception of tonal correctness, rythm, timbre, balance. The biggest problem is that with time your ears will adapt slowly to the distortion to compensate it and instead of compounding on your musical experiences you will become less and less interested and bored by it. You will want more and more chili... to make it fun.
Adding a sub on "high level" as you suggest will make things much worse. I suspect it is a move in the wrong direction.
Techeteri....
Buy A budget Nad amp and you have no choice about tonal colouration as they add bass without a loudness switch (by stealth) !
At least with the PM6004 you can have a choice to listen to a more balaned presentation or you can use the loudness which will give that NAD sound :rofl:
Seriously the PM6004 is one of the better budget amps out there, yours is faulty, change it, or wash your ears out